Nuclear fuel elements



United States Patent 3,331,748 NUCLEAR FUEL ELEMENTS Melville A.Faraday, Deep River, Ontario, Canada, as-

signor to Atomic Energy of Canada Limited, Ottawa, Ontario, Canada, acorporation of Canada No Drawing. Filed Aug. 9, 1965, Ser. No. 478,466 9Claims. (Cl. 17670) This invention relates to fuel elements for use innuclear reactors, particularly power reactors. A corrosionresistant,restrained, uranium-based fuel element is described, suitable for use inpressurized water, fog, or steam-cooled reactors to high burn-ups. Anadaptation is described for organic-cooled reactors.

The fuel elements comprise an inner core of adjusted uranium metal (a),surrounded by and bonded to a uranium alloy (b), which is in turn cladin a sheath of a zirconium alloy (c). A central void space is providedin the metal core (a) with both the U alloy (b) and Zr alloy (c)providing restraint on the core such that any swelling is directed intothe void space.

The major problems associated with the use of low alloy contenturanium-based fuels in high temperature water are the poor corrosionresistance and the high degree of swelling under irradiation especiallyabove 550 C. Limited corrosion resistance has been attained in the pastby using bulk fuels containing fairly large amounts of alloying materiale.g. U10 wt. percent Mo. Reductions in swelling have been attained usinga relatively thick stainless steel sheath for restraint and a small voiddown the centre of the U alloy into which the fuel could swell. Neitherof these approaches is acceptable in reactors using natural uraniumbecause of neutron absorption by the large amounts of sheathing andalloying materials involved.

Recently resistance to swelling of unalloyed uranium has been describedwhen the uranium is adjusted with minor alloying additions of iron andaluminum or silicon (to less than about 2000 ppm. total) and heattreated. These minor additions do not significantly affect the neutroneconomy. High temperature strength and water corrosion resistance havebeen described for uranium alloys containing 20-60 wt. percent Zr.Aluminum alloys containing about 10 wt. percent uranium have been shownto have good ductility and good aqueous corrosion resistance.

According to the present invention, fuel elements having several layershave been designed to provide resistance to swelling, strength andcorrosion resistance where needed.

The inner core contains uranium metal adjusted with minor alloyingadditions of both iron and aluminum. The iron may range from about 200to about'500 p.p.rn., preferably about 400 p.p.m. The aluminum may rangefrom about 500 to about 1200 p.p.m., preferably about 700 ppm. Thisadjusted uranium metal core should be heat treated (beta quenched fromabout 750 C. and alpha annealed at about 550 C. for several hours)during fabrication of the fuel element to refine and randomize the grainstructure. The core is constructed so as to contain a central void spaceof volume about to about 15% based on the fuel element and depending onthe operat- 3,331,748 Patented July 18, 1967 ing conditions andcharacteristics. Preferably the void space is about 7% of the uraniumvolume.

Surrounding the uranium metal core, a middle layer comprising an uraniumalloy is provided to give increased corrosion resistance, strength andrestraint to the fuel. Two material concepts are considered for themiddle layer: (1) One in which the layer is quite strong but withlimited ductility, e.g. uranium with 30 to 60 wt. percent zirconium and(2) The second in which the layer is extremely ductile, e.g. Al5 to 15wt. percent U. By this means a two component fuel is obtained which hasbetter neutron economy than any known single component fuel of similarswelling and corrosion resistance. Suitable uranium alloys contain anelement selected from the group consisting of Zr, Mo, Nb and Al. Zr andA1 are the preferred elements and the concentrations required are about30 to 60 wt. percent zirconium or 50 to percent aluminum. Theconcentrations of the Mo and Nb alloys are restricted to about 1 to 25wt. percent (preferably 1 to 10) because of their higher nuclearcross-section. The amount of the alloy, its concentration and the layerthickness may vary widely depending on the application, and on thenuclear, physical and corrosion characteristics of the alloy.

Depending on the application the amount of the U alloy may varyconsiderably. For small diameter fuel rods the uranium alloy layer isusually about l530 vol. percent of the metal core and the layerthickness about 0.1 cm. However, since the change in electrical energycosts with layer thickness is small, soundness in design will dictatelayer thickness. The ends of the uranium core are also bonded to alloydiscs or end plates to complete the shell.

For service where dimensional stability is required, but where watercorrosion resistance is not important, e.g. in an organic cooledreactor, a modified design may be used in which strengthening of theouter part of the adjusted uranium fuel is achieved by using alloyswhich are not highly Water corrosion-resistant or by dispersionstrengthening a surface layer of the uranium with powder or whiskers ofnon-metals. Examples of the non- .corrosion-resistant alloys which maybe used are U alloys containing low concentrations of one of Mo, Nb (1-3wt. percent) and A1 (5 30 wt. percent). The dispersion strengthening maybe achieved by the addition of certain non-metals, for example alumina.

An outer cladding or sheath is normally provided for corrosionresistance and added strength. The outer cladding is desirably acorrosion-resistant zirconium alloy, preferably Zircaloy-Z whichconsists of tin 1.2 to 1.7 wt. percent, iron 0.07 to 0.2 wt. percent,chromium 0.05 to 0.15 wt. percent, nickel 0.03 to 0.08 wt. percent,oxygen 1400 ppm. maximum, total Fe+Cr+Ni 0.18 to 0.38

water is good, but allows limited corrosion to provide a monitoringsignal that a defect has occurred. The corrosion rates of U-45 wt.percent Zr and All wt. percent U in 300 C. water are about 0.1 and 50mg./cm. /hr. respectively compared to 10 mg./ cm. hr. for uranium metal.The external diameter of the fuel element, the thickness of therestraining shell and the size of the central void can vary considerablydepending on the requirements of the particular reactor. Lowercompetitive neutron absorption is obtained than could be attained with asingle component fuel having similar corrosion and swelling behaviour.

The fuel elements can be made by one or more of the following methods:

(1) Single temperature or multi-temperature co-extrusion. This method isthe most attratcive one economically and technically.

(2) Individual machining of sections and shrink fit assembly. Intimatemechanical contact will be provided in this manner which will minimizethe central uranium temperature.

(3) Individual machining of sections and diffusion bond assembly. Thisdiffusion bonding treatment serves to reduce interface temperaturegradients to a minimum.

(4) Co-extruding the alloy shell and sheath and casting a central coreof uranium into that assembly. This method will produce bonded fuelswithout requiring multi-temperature extrusions.

(5) Extrusion clad the alloy shell onto the metal core (a); thisassembly is then slip fitted into the zirconium alloy sheath.

The naturally-occurring mixture of uranium isotopes is normally used,although enriched uranium can be used in either or both of the fuellayers, if desired.

Preferred embodiments are given in the following examples:

(1) Uranium metal is adjusted by the addition of 400 p.p.m. Fe and 700p.p.m. Al to the melt. The adjusted metal is co-extruded with U-45 wt.percent Zr alloy and a Zircaloy-2 sheath at about 650 C. The extrusionshould be heated to the beta region (about 750 C.), water quenched andannealed at about 550 C. for several hours in order to randomize andrefine the uranium structure, precipitate out some of the adjustingadditions, and epsilonize the uranium zirconium.

(2) Uranium metal is adjusted by the addition of 400 p.p.m. Fe and 700p.p.m. Al to the melt. The adjusted metal is then heated to the betaregion (about 750 C.), water quenched and annealed at about 550 C. forseveral hours to randomize and refine the uranium structure andprecipitate out some of the adjusting additions. The uranium is thenextrusion clad in Al wt. percent U at about 525 C.; this rod is thenslip fitted inside a Zircaloy sheath.

The final fuel rod diameter is 1.52 cm. while the diameters of the void,inner uranium core and uranium alloy outer layers are approximatley 0.4,1.28 and 1.45 cm. respectively. The fuel rods are cut to 19 incheslength and arranged to give a 19 element bundle (as for the NPD Rolphtonreactor). Bundles of 22 or 28 elements of smaller diameter would be evenmore attractive. The metallurgical bonds between the uranium core andthe uranium alloy layers in these examples are at least as good asbetween uranium and Zircaloy-2. The uranium alloy layer may or may notbe bonded to the Zircaloy-2 sheath, depending on the application.

(3) URANIUMZIRCONIUM (a) Test specimens have been made by arc castinguranium metal into an outer shell of uranium-47 wt. percent zirconium toproduce a metallurgical bond. The open end was sealed by welding a plugof U-47 wt. percent Zr onto the outer shell of U-47 wt. percent Zr usinga technique developed for welding Zircaloy-2. This specimen was thenslip-fitted inside a Zircaloy-2 sheath and Zircaloy-2 end plugs wereWelded on to produce a sealed element (5 cm. long x 2 cm. diameter).

(b) Specimens of the U-47 wt. percent Zr of similar size'were corrosiontested in 300 C. water both bare and clad in a 0.060 cm. thick defectedZircaloy-2 can in 300 C. water. The bare U-47 wt. percent Zr corrodeduniformly without any pitting and at a rate of about 0.1 mg./ cm. hr.The specimen clad in the defected Zircaloy-2 can showed little visiblechange after 360 hours at 300 C. Dimensionally, the can had swelled byabout 0.001 cm., but no signs of hydriding could be found in theZircaloy-2.

(4) ALUMINUM-URANIUM A sample (4.5 cm. long x 0.5 cm. diameter) of All0wt. percent U tested bare in 300 C. water for thirty minutes had acorrosion rate of about 50 mg./ cm. hr. A four hour defect test in 300C. water of a similar sample clad in 0.030 cm. thick Zircaloy-2 resultedin a short split and a slight bulge in the sheath. Examination of across section of this element indicated that a less than 0.1 cm. thickshell of Al-l0 wt. percent U is adequate to give about four hoursprotection in embodiment No. 2 above.

I claim:

1. A fuel element for nuclear reactors comprising:

(a) an inner layer of uranium metal containing from 200 to 500 p.p.m.iron, from 500 to 1200 p.p.m. aluminum, and beta quenched and alphaannealed to refine and randomize the grain structure, and enclosing acentral void space,

(b) a middle layer comprising one of (1) an uranium alloy containing anelement selected from the group consisting of Zr, Mo, Nb and Al, the Zrbeing present in from 30 to 60 wt. percent, the Mo and Nb in from 1 to25 wt. percent, and the Al in from 50 to 95 wt. percent, and (2) uraniumdispersion-strengthened with alumina, and

(c) an outer sheath of a corrosion-resistant zirconium alloy.

2. A fuel element for nuclear reactors comprising:

(a) an inner layer of uranium metal containing about 400 p.p.m. iron andabout 700 p.p.m. aluminum, and beta quenched from about 750C. and alphaannealed at about 550 C., and enclosing a central void space,

(b) a middle layer of an uranium alloy containing from 30 to 60 wt.percent of Zr, and

(c) an outer sheath of a corrosion-resistant zirconium alloy.

3. A fuel element for nuclear reactors comprising:

(a) a central void space surrounded by an inner layer of uranium metalcontaining about 400 p.p.m. iron, about 700 p.p.m. aluminum and betaquenched from about 750 C. and alpha annealed at about 550 C.,

(b) a middle layer of uranium-45 to 47 wt. percent zirconium alloy, and

(c) an outer sheath of the zirconium alloy Zircaloy-2.

4. A fuel element for nuclear reactors comprising:

(a) a central void space surrounded by an inner layer of uranium metalcontaining about 400 p.p.m. iron, about 700 p.p.m. aluminum and betaquenched from about 750 C. and alpha annealed at about 550 C.,

(b) a middle layer of aluminum-l0 wt. percent uranium, and

(c) an outer sheath of the zirconium alloy Zircaloy-2.

5. The fuel element of claim 1 wherein the uranium alloy in middle layer(b) contains one of from 1 to 10 wt. percent Mo, from 1 to 10 wt.percent Nb, and from to wt. percent Al.

6. The fuel element of claim 1 wherein the void space is about 5 to 15vol. percent of the fuel element.

7. The fuel element of claim 1, including end plates of the middle layer(b) bonded to the inner layer (a), and to middle layer (b).

8. The fuel element of claim 1, wherein the middle layer (b) is about15-30 vol.

percent of the inner layer 9: The fuel element of claim 1, in a bundleof from 19 to 28 elements. 5

11/1959 McGeary et al. 176-89 X 12/1959 Saller 17689 X 6 McGeary et a117689 X Jepson et a1 76122.7 X Precht et a1. 176-89 X Maxwell 17669Wyatt et al. 264.5 X Market et al. 17691 X Lustman et al 176-67 XBellamy 176-70 X CARL D. QUARFORTH, Primary Examiner. BENJAMIN R.PADGETT, Examiner. N. J. SCOLNICK, Assistant Examiner.

1. A FUEL ELEMENT FOR NUCLEAR REACTORS COMPRISING: (A) AN INNER LAYER OFURANIUM METAL CONTAINING FROM 200 TO 500 P.P.M. IRON, FROM 500 TO 1200P.P.M. ALUMINUM, AND BETA QUENCHED AND ALPHA ANNEALED TO REFINE ANDRANDOMIZED THE GRAIN STRUCTURE, AND ENCLOSING A CENTRAL VOID SPACE, (B)A MIDDLE LAYER COMPRISING ONE OF (1) AN URANIUM ALLOY CONTAINING ANELEMENT SELECTED FROM THE GROUP CONSISTING OF ZR, MO, NB AND AL, THE ZRBEING PRESENT IN FROM 30 TO 60 WT. PERCENT, THE MO AN NB IN FROM 1 TO 25WT. PERCENT, AND THE AL IN FROM 50 TO 95 WT. PERCENT, AND (2) URANIUMDISPERSION-STRENGTHENED WITH ALUMINA, AND (C) AN OUTER SHEATH OF ACORROSION-RESISTANT ZIRCONIUM ALLOY.